County health officials re-opened a half-mile stretch of Aliso Beach on Wednesday after a two-day closure due to a 9,500-gallon sewage spill, the second largest spill locally in six years.

Looking south towards Aliso Beach, which reopened on Wednesday after a closure.

Closure signs were removed following two consecutive days of testing to determine if water spilling from Aliso Creek meets state water-quality standards, said Larry Honeybourne, assistant director of the county health agency’s environmental health division.

The spill nears the 10,000 gallons dumped into Three Arch Bay in May 2011 due to construction debris, but pales compared to Laguna’s largest sewage catastrophe when 591,000 gallons of raw sewage spewed onto Bluebird Beach and resulted in a nearly citywide closure for six days, county records show. Beaches were also closed in December 2010 due to widespread flooding.

Raw sewage flowed into Aliso Creek upstream from Laguna Hills due to a blocked siphon at 3 a.m. Monday, April 28, said Matt Collings, assistant general manager of the Moulton Niguel Water District.

Testing at one of the county’s most popular beaches did not determine if the outflow from La Paz Road and Moulton Parkway actually reached the sand six miles downstream, but health officials closed the beach temporarily as a precaution. The closure spanned a quarter mile on either side of the creek mouth, including the primary cove below the Montage resort.

Water is tested for indicator organisms that are naturally present but would be elevated if sewage was present, Honeybourne said. He figures a small amount of sewage likely did reach the surf zone, but testers didn’t pick up unsafe levels on Wednesday. Hot weather, dilution in creek water and predator organisms neutralize the organisms, he said.

Monday’s spill was the second of the year for Moulton, which serves 168,000 people in five mostly inland south-county cities. On Feb. 12, a mechanical failure at a sewage lift station operated by Moulton dumped 1,500 gallons of sewage into Salt Creek, county records show.

Unspecified debris apparently clogged a siphon and water district officials are investigating the source. Water district workers tried to prevent some of the spill from reaching the ocean by using a vacuum-device to suck ponding sewage from La Paz Regional Park on Monday, said Collings.

Plastic bags and even beach towels flushed down toilets have led to spills, said David Shissler, Laguna’s director of water quality.

Since the siphon is cleaned monthly, water district officials are mystified about the source of the debris. The spill was reported to the San Diego Regional Water Quality Control Board and be subject to a fine for violating water-quality regulations.

Countywide, beach-goers were turned away from the sand due to sewage spills for 11 beach miles in 2013, compared to the peak of 175 beach mile day closures in 2001, according to OC Health Care Agency reports.